I2O 



SMALL WATER SUPPLIES. 



from time to time in schemes of estate water supply 

 as will be pointed out. " a " signifies the natural 

 angle of slope of the particular earth in question, 

 which will be found in pocket-books of engineering 

 matter. Generally for ordinary earth the quantity 



tan ~ will be '414 and for stiff clay 767, while for 



water it is unity, and in the same way it will denote 

 the weight of i cub. ft. of the earth in question. 

 Let us consider the case of a small masonry dam, 



straight in plan, built of 

 granite in cement mor- 

 tar, and of the section 

 shown in fig. 86. A B 

 is assumed to be a single 

 bed joint, the friction on 

 which will prevent fail- 

 ure by sliding. It may 

 here be remarked that a 

 wall may fail by 



1. Shearing. 



2. Toppling over. 



3. Crushing on the 

 outer joints and conse- 

 quent separation of the 

 inner ones. 



FIG. 86. 



4. Sliding on the surface. 



5. Sliding on the bed joints. 



In fig. 86 C D is the water level. At the points A 

 and B it will be necessary to find the pressure due to 

 the water at A and tension at B. The masonry 

 which is heavy would weigh 170 Ib. per cubic foot. 

 The area of the cross-section of the wall is as per 



